Where does Blender 2.8 store theme files on Mac OS?
You can use this utility file to get the theme file 

blender-build/blender/source/tools/utils/blender_theme_as_c.py



  Generates 'userdef_default_theme.c' from a 'userpref.blend' file.
  
  Pass your user preferenes blend file to this script to update the C source file.
  
  eg:

./source/tools/utils/blender_theme_as_c.py ~/.config/blender/2.80/config/userpref.blend